1. Field of the Invention
The present invention relates to a fixing device mounted on an image forming apparatus, such as a copying machine and a printer, in which an electrophotographic technology is used.
2. Description of the Related Art
A fixing device in which a film is used is known as the fixing device mounted on the image forming apparatus such as the copying machine and the printer. Typically, the fixing device includes a cylindrical film, a plate-like heater that contacts with an inner surface of the film, and a roller that constitutes a nip portion together with the heater via the film. In fixing processing of the fixing device, a recording material on which a toner image is formed at the nip portion is heated while being conveyed, whereby the toner image is fixed to the recording material. In the fixing device, since the film having a low heat capacity is used, there is a merit of shortening a warming-up time of the fixing device and contributing to the shortening of First Print Out Time (FPOT) of the image forming apparatus.
Nowadays, there is an increasing need for downsizing the image forming apparatus, and it is conceivable that the downsizing of the fixing device is achieved by further decreasing a diameter of the film or roller. However, in order to decrease the diameter of the film, it is necessary to narrow a width of the heater in a recording material conveyance direction. Therefore, Japanese Patent Application Laid-Open No. 2003-337484 discusses a fixing device including a heater in which a heat generation amount distribution can be formed according to the width of the recording material by separately disposing heating resistors having different lengths in both surfaces of a substrate of the heater even if the width of the heater is narrow. The fixing device discussed in Japanese Patent Application Laid-Open No. 2003-337484 includes a temperature detection unit for detecting a temperature at a surface on an opposite side to a surface contacting the film of the heater, and power supplied to the heating resistors provided on both the surfaces of the heater is controlled so that the temperature detected by the temperature detection unit becomes a target temperature.
However, the following problem is generated if a target temperature is the same between when the power is supplied to the heating resistor formed on one of the surfaces of the heater and when the power is supplied to the heating resistor formed on the other surface.
Sometimes a thermal resistance of a heat conduction path to the temperature detection unit from the heating resistor formed on the surface contacting with the film of the heater differs from a thermal resistance of a heat conduction path to the temperature detection unit from the heating resistor formed on the surface on the opposite side to the surface contacting with the film of the heater. When the same target temperature is set for a surface of the heater without determining to which surface the power is supplied, the surface temperature at the film differs, which results in a problem in that a fixing defect is generated.